1. Technical Field of the Invention
The present invention relates to an orthogonal modulator for removing a DC offset component of each of a plurality of base band signals, a mobile terminal comprising the orthogonal modulator, and a communication system including such mobile terminals.
2. Description of the Prior Art
Conventionally, a mobile terminal such as a cellular phone has employed the orthogonal modulation by which a digital voice signal (base band signal) is transmitted, for example, by two carrier waves having different phases by 90xc2x0.
More specifically, the conventional mobile terminal amplifies and modulates an I channel base band signal (I signal), a Q channel base band signal (Q signal) with a carrier wave having an orthogonal relationship by means of the orthogonal modulator and then adds them, thereby obtaining a modulated output signal.
However, in the case in which the orthogonal modulator or the base band signal input thereto has DC offset, the carrier leak appears due to the DC offset over a modulated wave obtained by the orthogonal modulator. Therefore, there have been developed several techniques for suppressing the carrier leak.
The conventional orthogonal modulator as shown in FIG. 5 comprises base band LSI 81 for generating an I signal and a Q signal and an IB signal and a QB signal of which phases are inverted thereto by 90xc2x0 phase shifter 83 for modulating the phase of the output carrier wave source 84, and I/Q mixer 82 such as a Gilbert multiplier for mixing the output signal of the base band LSI 81 and the output signal of the 90xc2x0 phase shifter 83 through filter 85.
The operation of the orthogonal modulator shown in FIG. 5 is explained below. The base band LSI 81 inputs an I/Q DC level offset setting signal from non-shown ROM, and outputs an I signal, a Q signal, an IB signal and a QB signal. The IB signal and the QB signal are generated in order to operate the I/Q mixer 82 in good balance.
Moreover, the I/Q DC level offset setting signal is decided so as to minimize the carrier leak of an MOD signal (modulated wave) outputted from an MOD terminal of the I/Q mixer 82. In other words, the carrier leak is suppressed in response to the I/Q DC level offset setting signal.
A pair of I and IB base band signals and a pair of Q and QB base band signals are inputted into transistors Q1 to Q4 of the I/Q mixer 82 through the filter 85.
Moreover, transistors Q5 to Q12 of the I/Q mixer 82 input, in a predetermined configuration, a 0xc2x0 carrier wave, a 90xc2x0 carrier wave, a 180xc2x0 carrier wave and a 270xc2x0 carrier wave which are outputted from the carrier wave source 84 through the 90xc2x0 phase shifter 83.
These carrier waves and each base band signal are mixed and are transmitted from MOD terminal to the air. Thus, the carrier leak of the MOD signal is minimized.
FIG. 6 is a block diagram of a carrier leak suppressing circuit as an orthogonal modulator disclosed in JP 6-303145 A, 1994. Orthogonal modulation unit 100 inputs the I and Q signals having phases different from each other by 90xc2x0 and outputs an orthogonal modulating signal. A part of the orthogonal modulating signal is inputted into demodulation unit 101. The demodulation unit 101 inputs a carrier Lo outputted from the orthogonal modulation unit 100.
The demodulation unit 101 demodulates the inputted signal into the I and Q signals. The DC offset component of the I signal and the DC offset component of the Q signal thus demodulated are fed back to the orthogonal modulation unit 100 in order to remove the DC offset components from the I and Q signals. Thus, the carrier leak of the modulated wave is suppressed.
A specific structure of carrier leak suppressing circuit as shown in FIG. 6 is shown in FIG. 7.
The orthogonal modulation unit 100xe2x80x2 modulates the I signal, while the orthogonal modulation unit 100xe2x80x3 modulates the Q signal. Further, demodulation unit 101xe2x80x2 demodulates the I signal, while demodulation unit 101xe2x80x3 demodulates the Q signal.
Each of the I and Q signals is converted from a digital signal into an analog signal by D/A converters 111 and 121, is subjected to a predetermined processing in operational amplifiers 112 and 122 and roll-off filters 113 and 123, and is outputted into mixers 114 and 124 together with carrier waves 0xc2x0 and 90xc2x0 which are the outputs of a local oscillator 120.
Then, the I and Q signals and the orthogonal carrier wave are mixed and amplitude-modulated. Thus, an orthogonal modulating signal is outputted from the antenna. A part of the orthogonal modulating signal is inputted into demodulation units 101xe2x80x2 and 101xe2x80x3. The offset components are extracted by band pass filters 101A and 102A, tuned to the timing of the carrier on the modulation side by means of delay elements 101L and 102L and are outputted into demodulating mixers 101X and 102X.
The demodulating mixers 101X and 102X also input the orthogonal carrier waves 0xc2x0 and 90xc2x0, mix the output signals of the delay elements 101L and 102L and the orthogonal phase carrier waves, thereby detecting DC offset components. The DC offset component is integrated by low-pass filters 101F and 102F and is fed back to the operational amplifiers 112 and 122 through amplifiers 101a and 102a. 
Each of the operational amplifiers 112 and 122 calculates a difference between the output signal of the D/A converter and the output signals 101a and 102a and remove the DC offset components. A base band signal from which the DC offset component is removed is outputted into composite hybrid 130 through the roll-off filters 113 and 123 and the mixers 114 and 124. The output from the mixer 114 and the output signal from the mixer 124 are mixed and the mixed signal is outputted to the outside of the orthogonal modulator through the band pass filter 140.
In the conventional orthogonal modulator as shown in FIG. 5, the base band LSI generates each base band signal in response to the I/Q DC level offset setting signal output from the ROM. However, the carrier leak of the modulated wave obtained from the base band signal cannot be always decreased. The DC offset between the I and Q signals of the base band LSI and mixer circuit does not always have a mutual relationship, due to variations in manufacturing processes and environmental conditions such as an ambient temperature, or light.
Moreover, the set value of the I/Q DC level offset cannot be changed because it is stored in the ROM. For this reason, it is impossible to carry out correction corresponding to a variation in the DC offset between the I and Q signals of the base band LSI and mixer circuit and the like.
Furthermore, in the conventional orthogonal modulator as shown in FIG. 5, when the DC levels of the I and Q signals outputted from the base band LSI are lowered, the voltage between the collector and the emitter of the transistor of the I/Q mixer is also lowered, so that a constant current does not flow to the I/Q mixer. Consequently, the modulating signal loses its linearity with respect to the I/Q signal and an I/Q tertiary modulation distortion is increased.
In the conventional orthogonal modulator as shown in FIG. 6, the modulated signal is demodulated into the I and Q signals and the DC offset component of the I signal and the DC offset component of the Q signal are fed back to the orthogonal modulation unit. However, there is no feedback loop for removing the DC offset component between the I signal and the Q signal. For this reason, it is impossible to remove the carrier leak by the DC offset component between the I signal and the Q signal.
Moreover, the power consumption of the demodulation unit is almost equivalent to that of the orthogonal modulation unit. Accordingly, the power consumption as a whole circuit as shown in FIG. 6 is increased to almost a double as compared with the orthogonal modulator according to the circuit as shown in FIG. 5.
Further, in the conventional orthogonal modulator as shown in FIG. 7, it is difficult to design an accurate band pass filter suitable for an integrated circuit.
Therefore, an object of the present invention is to provide an orthogonal modulator capable of removing the DC offset component between the I signal and the Q signal.
Another object of the present invention is to prevent an increase in the tertiary modulation distortion of the I signal and the Q signal.
The orthogonal modulator of the present invention comprises: a generation means for generating a plurality of base band signals from a transmitted signal such as a voice signal; a mixing means for mixing the base band signals and a plurality of carrier waves; an extraction means for extracting each DC offset component generated by the generation means or the mixing means in response to the base band signals; a comparison means for comparing the DC offset components thus extracted with each other; an addition means for adding a plurality of comparison result signals obtained as a result of the comparison to other comparison result signals; and an offset elimination means for eliminationg the DC offset components included in a plurality of addition signals obtained as a result of the addition from the base band signals.
Moreover, the present invention provides a mobile terminal comprising voice collecting means for collecting a voice generated by a user, the orthogonal modulator for inputting and orthogonally modulating a voice signal output from the voice collecting means, and transmitting means for transmitting an orthogonal modulating signal output from the orthogonal modulator.
Furthermore, the present invention provides a communication system comprising the mobile terminal, and a base station for transmitting and receiving electric waves to and from the mobile terminal.
According to the present invention, as described above, in the case in which a plurality of base band signals and a plurality of carrier waves orthogonal thereto are to be mixed, the DC offset components of the base band signals are extracted from the mixing means and are fed back and removed from the base band signals. Accordingly, it is possible to provide the orthogonal modulator capable of removing the offset component between the I signal and the Q signal.